The present invention relates generally to devices and methods of measuring and indicating applied bolt loads, and, more particularly, to a mechanically actuated bolt load indicator having variable, adjustable indicator amplification which provides easy calibration independent of a fastener and more accurate bolt load measurement and indication thereof.
Mechanical bolt load indicating devices are used where it is necessary to continuously monitor the tensile strain that a bolt or other fastener is subjected to while in use. Such load indicating apparatuses are typically installed into a recessed portion of a bolt head such that a mechanical load indicator element is caused to move relative to the amount the bolt stretches or elongates as a load is applied thereto. A dial plate having a reference scale is used to indicate to the user the amount of load to which the bolt is being subjected when it is initially tightened and thereafter as the load changes over time.
Prior art examples of mechanical bolt load indicating apparatuses include Maxbolt™ and Betabolt™ load-indicators. The structure and operation of the Maxbolt™ and Betabolt™ load indicators are described in detail, respectively, in U.S. Pat. Nos. 5,668,323 and 5,584,627 the disclosures of which are incorporated herein by reference in their entirety.
FIGS. 1 and 2 show a prior art mechanically actuated bolt load indicator and fastener 70 composed of a body 72 having a shank 74 and an abutment 60A formed at the end of a gauge pin 60, which is moved by the changing length of the body 72 under an applied load, and a load sensing member 11. The body is drilled with a blind bore 76. The fastener may be used, for example, as shown to fasten two flat members 80 together using the nut 78. The load sensing member 11 is composed of an element 51 disposed in the direction in which the load is applied when the fastener is in use. The element 51 is of elongated U-shape, a first limb 51 C of the U rigidly attached to the body 72, a closed end 51 A of the U bearing on the abutment 60A so as to move with the abutment 60A, and a second limb 51 B of the U having registering the applied load 5 on a scale 17 (FIG. 2). FIG. 18 describes a process of making this fastener.
Indicator needles in conventional mechanical load indicators, such as that of FIGS. 1 and 2, provide a fixed and limited amount of amplified movement relative to the amount that the bolt stretches when subjected to a load. Therefore, the sweep distance of the indicator needle from a zero reference to a maximum reference is relatively small thereby decreasing the overall precision with which the device can be read because the dial plate is relatively small with few well defined bolt load references. For example, a typical bolt load that causes a 0.004″ elongation of a bolt produces a mere 0.125″ total sweep distance of the indicator needle from a zero reference to a maximum reference.
Another problem that exists with conventional bolt load indicators is that their relatively small dial plates may be difficult to see and read correctly due to the parallax effect. Adverse environmental conditions such as poor lighting, moisture, debris or the location of the bolt in which the load indicator is installed may make it even more difficult to read the indicator correctly.
A still further problem with conventional bolt load indicators is that conventional indicator needles travel or move in a generally linear direction across the dial plate. Therefore, the maximum needle moment amplification that is obtainable is limited to the diameter of the dial plate which in turn is limited by the size of the bolt head itself
A still further problem that exists with conventional bolt load indicators is that they can not be universally calibrated during the assembly process.